Use of bimodal carbon distribution in compacts for producing metallic iron nodules

ABSTRACT

A method for use in production of metallic iron nodules comprising providing a reducible mixture into a hearth furnace for the production of metallic iron nodules, where the reducible mixture comprises a quantity of reducible iron bearing material, a quantity of first carbonaceous reducing material of a size less than about 28 mesh of an amount between about 65 percent and about 95 percent of a stoichiometric amount necessary for complete iron reduction of the reducible iron bearing material, and a quantity of second carbonaceous reducing material with an average particle size greater than average particle size of the first carbonaceous reducing material and a size between about 3 mesh and about 48 mesh of an amount between about 20 percent and about 60 percent of a stoichiometric amount of necessary for complete iron reduction of the reducible iron bearing material.

This application is a continuation of and claims the benefit of U.S.patent application Ser. No. 12/977,035, filed on Dec. 22, 2010, now U.S.Pat. No. 8,287,621, issued on Oct. 16, 2012, the disclosure of which isincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

The present invention was made with support by the Department of Energy,Sponsor Award DE-FG36-05GO15185. The United States government hascertain rights in the invention.

BACKGROUND AND SUMMARY

The present invention relates to reduction of iron bearing materialssuch as iron ore to metallic iron nodules (known as “NRI”).

Metallic iron has been produced by reducing iron oxide such as ironores, iron pellets, and other iron sources. Various such methods havebeen proposed so far for directly producing metallic iron from iron oresor iron oxide pellets by using reducing agents such as coal or othercarbonaceous material. Such fusion reduction processes generally involvethe following processing steps: feed preparation, drying, preheating,reduction, fusion/melting, cooling, product discharge, and metalliciron/slag product separation. These processes result in direct reductionof iron bearing material to metallic iron nodules (NRI) and slag.Metallic iron nodules produced by these direct reduction processes arecharacterized by near total reduction, approaching 100% metal (e.g.,about 96% or more metallic Fe). Percents (%) herein are percents byweight unless otherwise stated.

Unlike conventional direct reduced iron (DRI) product, the metallic ironnodule (NRI) product has little or no gangue and little or no porosity.NRI is essentially metallic iron product desirable for manyapplications, such as use in place of scrap in steelmaking by electricarc furnaces. Metallic iron nodules are generally as easy to handle astaconite pellets and DRI, and are a more efficient and effectivesubstitute for scrap in steel making by electric arc furnace (EAF)without extending heat times and increasing energy cost in making steel.

Various types of hearth furnaces have been described and used for directreduction of NRI. One type of hearth furnace used to make NRI is arotary hearth furnace (RHF). The rotary hearth furnace is partitionedannularly into temperature zones between a supply location and thedischarge location of the furnace. An annular hearth is supportedrotationally in the furnace to move from zone to zone carrying reduciblematerial the successive zones to reduce and fuse the reducible materialinto metallic iron nodules, using one or more heating sources (e.g.,natural gas burners). The reduced and fused NRI product, aftercompletion of the process, is cooled to prevent reoxidation andfacilitate discharge from the furnace. Another type of furnace used formaking NRI is the linear hearth furnace such as described in U.S. Pat.No. 7,413,592, where similarly prepared mixtures of reducible materialare moved on moving hearth sections or cars through a drying/preheatingzone, a reduction zone, a fusion zone, and a cooling zone, between thecharging end and discharging end of a linear furnace while being heatedabove the melting point of iron. As one example, a method for use inproduction of metallic iron nodules is disclosed in U.S. Pat. No.7,628,839.

It has been desired in the production of NRI to reduce the amount oftime for reduction and fusion of reducible material in forming metalliciron nodules while reducing the amount of sulfur in the nodules andlimiting the formation of micro metallic iron nodules. Micro metalliciron nodules (called micro-nodules or micro NRI) include small particlesof agglomerated iron having a size between about 20 mesh and about 3mesh.

What is disclosed is a method for use in production of metallic ironnodules comprising the steps of

-   -   providing a hearth comprising refractory material;    -   providing reducible mixture above at least a portion of the        refractory material, the reducible mixture comprising at least        reducing material and reducible iron bearing material;    -   forming the reducible mixture to comprise:        -   a quantity of reducible iron bearing material,    -   forming the reducible mixture to comprise:        -   a quantity of reducible iron bearing material,        -   a quantity of first carbonaceous reducing material of a size            less than about 48 mesh of an amount between about 65            percent and about 95 percent of a stoichiometric amount            necessary for complete iron reduction of the reducible iron            bearing material, and        -   a quantity of second carbonaceous reducing material with an            average particle size greater than average particle size of            the first carbonaceous reducing material and a size between            about 3 mesh and about 48 mesh of an amount between about 20            percent and about 65 percent of a stoichiometric amount of            necessary for complete iron reduction of the reducible iron            bearing material;        -   where amount of first carbonaceous reducing material and            second carbonaceous reducing material provide total reducing            material carbon between about 110 and 150 percent of a            stoichiometric amount necessary for complete iron reduction            of the reducible iron bearing material, and    -   thermally treating the reducible mixture in the presence of        other carbonaceous material separate from the reducible mixture        to form one or more metallic iron nodules by melting.

The quantity of first carbonaceous reducing material may be of an amountbetween about 80 percent and about 90 percent of a stoichiometric amountnecessary for complete iron reduction of the reducible iron bearingmaterial. Alternatively, the quantity of first carbonaceous reducingmaterial may be of an amount between about 85 percent and about 95percent of a stoichiometric amount necessary for complete iron reductionof the reducible iron bearing material. In yet another alternative, thequantity of first carbonaceous reducing material may be of an amountbetween about 65 percent and about 75 percent of a stoichiometric amountnecessary for complete iron reduction of the reducible iron bearingmaterial. The quantity of second carbonaceous reducing material being ofan amount between about 20 percent and about 50 percent of astoichiometric amount necessary for complete iron reduction of thereducible iron bearing material.

The basicity B₂ of the reducible mixture may be between 1.5 and 2.3.Alternatively, the basicity B₂ of the reducible mixture is between 1.9and 2.3.

The first carbonaceous reducing material may be of a size less thanabout 65 mesh. Alternatively, the first carbonaceous reducing materialmay be between about 65 mesh and about 100 mesh.

The second carbonaceous reducing material may be of a size between about6 mesh and about 65 mesh. Alternatively, the second carbonaceousreducing material may be of a size between about 6 mesh and about 48mesh.

The first carbonaceous reducing material may include at least twosources of carbonaceous material, at least one source being fines lessthan about 48 mesh from a source of carbonaceous material in the secondcarbonaceous reducing material.

The first carbonaceous reducing material may be a carbonaceous materialwith between 2 and 40% average volatiles, and the second carbonaceousreducing material may be a non-caking carbonaceous material with lessthan 10% average volatiles. Alternatively, the second reducing materialmay be a non-caking carbonaceous material with between 1 and 8%volatiles.

The reducible mixture may be formed into agglomerates. In onealternative, the second carbonaceous reducing material is of a size lessthan 20 mesh and the reducible mixture is formed into balls.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of embodiments of the present methodcan be best understood when read in conjunction with the followingdrawings, where like structure is indicated with like reference numeralsand in which:

FIG. 1 is a cross sectional diagrammatical view showing a hearth furnacefor producing metallic iron material,

FIG. 2 is a generally top view showing metallic iron nodules above ahearth,

FIG. 3 is a generalized cross-sectional view showing a hearth and thelayers thereon, and

FIG. 4 is a table of chemical compositions of one or more additives thatmay be used in one or more embodiments of the metallic iron noduleprocesses described herein,

FIG. 5 is a table showing compositions of material samples used informing a reducible mixture for making metallic iron nodules by thepresent method,

FIG. 6 is a table showing proximate analysis of a high-volatilebituminous coal,

FIG. 7 is a table of size distribution of mill scale as received priorto forming a reducible mixture,

FIG. 8 is a table of compositions of the reducible mixtures of varyingamounts of carbonaceous material for making metallic iron nodules by oneembodiment of the present method,

FIG. 9 is a table of compositions of the reducible mixtures of varyingamounts of bimodal carbonaceous materials for making metallic ironnodules by an alternative embodiment of the present method,

FIG. 10 is a table of compositions of the reducible mixtures of varyingamounts of bimodal carbonaceous materials and varying basicity formaking metallic iron nodules by an alternative embodiment of the presentmethod,

FIG. 11 is an image of various reducible mixtures prepared for makingmetallic iron nodules by the present method,

FIG. 12 is an image of the various reducible mixtures of FIG. 11 afterundergoing heating by the present method to form metallic iron nodules,

FIG. 13 is a table showing a partial sample of experimental testparameters producing metallic iron nodules using reducible mixtures ofFIG. 10,

FIG. 14 is a graph showing the effect of increasing carbonaceousreducing material on fusion time, sulfur and micro-nugget generation,and

FIG. 15 is a table of compositions of the reducible mixtures of varyingamounts of bimodal carbonaceous materials and varying basicity formaking metallic iron nodules by another alternative embodiment of thepresent method,

FIGS. 16A and 16B are tables of compositions of the reducible mixturesincluding taconite and varying basicity for making metallic iron nodulesby another alternative embodiment of the present method.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to FIGS. 1 through 3, a hearth furnace 60 for producingmetallic iron material directly from iron ore and other iron oxidesources may include a furnace housing 62 and a hearth 64. The furnacehousing 62 includes a furnace roof 66 and side walls 68 internally linedwith a refractory material suitable to withstand the temperaturesinvolved in the metallic reduction process carried out in the furnace.The hearth 64 may be any moving hearth suitable for use with the hearthfurnace 60 operable for production of metallic iron nodules 70.Generally, the hearth 64 includes refractory material upon which areducible mixture to be processed (e.g., feed material) is received. Thehearth 64 may be a hearth suitable for use in a rotary hearth furnace, alinear hearth furnace (e.g., as shown in FIGS. 1 and 2), or any otherfurnace system operable for production of metallic iron nodules 70(NRI).

The refractory material lining the interior of the furnace may be, forexample, refractory board, refractory brick, ceramic brick, or acastable refractory material. More than one refractory material may beused in different locations as desired. For example, a combination ofrefractory board and refractory brick may be selected to provideadditional thermal protection for any underlying substructure. Thehearth 64 may include a supporting substructure 72 that moves therefractory material (e.g., a refractory lined hearth) forming hearth 64through the furnace. The supporting substructure may be formed from oneor more different materials, such as, for example, stainless steel,carbon steel, or other metals, alloys, or combinations thereof that havesuitable high temperature characteristics for furnace operation.

The hearth furnace 60 may be divided into at least a conversion zone 74capable of providing a reducing atmosphere for the reducible material,and a fusion zone 76 capable of providing an atmosphere to at leastpartially form metallic iron material. A drying/preheating zone 78 maybe provided adjacent the furnace housing capable of providing adrying/preheating atmosphere for the reducible mixture. Additionally, acooling zone 80 capable of providing a cooling atmosphere for reducedmaterial containing metallic iron material may be provided in oradjacent the furnace housing immediately following the fusion zone 76.As noted, the cooling zone may be in the furnace housing 62, but asshown in FIG. 1, the cooling zone may be provided outside the furnacehousing since the furnace housing is not necessary to its operation.Also as noted, the drying/heating zone 78 may be provided inside oroutside the furnace housing in desired embodiments.

In any case, the conversion zone 74 is positioned between thedrying/preheating zone 78 and the fusion zone 76 and is the zone inwhich volatiles from the reducible mixture, including carbonaceousmaterial, are fluidized, as well as the zone in which at least theinitial reduction of metallic iron material occurs. The entry end of thehearth furnace 60, at the drying/preheating zone 78, may be at leastpartially closed by a restricting baffle 82 that may inhibit fluid flowbetween the outside ambient atmosphere and the atmosphere of thedrying/preheating zone 78, yet provide clearance so as not to inhibitthe movement of reducible mixture into the furnace housing 62. Thebaffle 82 may be made of suitable refractory material such as siliconcarbide or a metal material if the temperatures are sufficiently low.The atmosphere in the hearth furnace 60 is typically maintained at apositive pressure compared to the ambient atmosphere to further inhibitfluid flow from the ambient atmosphere to the hearth furnace. The methodof producing metallic iron nodules may include reducing the reduciblemixture in the hearth furnace 60 to metallic iron nodules substantiallyfree of air ingress from the surrounding environment.

The hearth 64 provided within the furnace housing 62 may comprise aseries of movable hearth cars 86 that are positioned contiguously end toend as they move through the furnace housing 62. Hearth cars 86 may bemovable on wheels 88 that engage rails 90. The upper portion of thehearth cars 86 are lined with a refractory material suitable towithstand the temperatures for reduction of the iron oxide bearingmaterial into metallic iron nodules as explained herein. The hearth carsare positioned contiguously end to end to form hearth 64 and movethrough the furnace housing 62, so that the lower portions of the hearthcars are not damaged by the heat generated in the furnace as reductionof the iron oxide-bearing material into metallic iron proceeds.Alternatively, the hearth 64 may be a movable belt or other suitableconveyance medium provided with refractory material for the temperaturesof the furnace atmospheres.

The hearth furnace may be linear as generally illustrated in FIGS. 1 and2. With a linear furnace, the building in which the furnace is housed,or other considerations, may require that certain parts of the furnacebe arcuate or at angles, to accommodate these needs. For these purposes,the hearth furnace is classified as linear if a part of its length,usually the conversion zone 74 and/or fusion zone 76, is substantiallylinear in the direction of travel of the hearth 64. Alternatively, thehearth furnace may be rotary, in which case the hearth cars arepie-shaped or in the form of replaceable sections of a contiguousannular hearth rotatably supported in the furnace housing.

The zones of the furnace 60 are generally characterized by thetemperature reached in each zone and the processing of the reduciblemixture in each zone. In the drying/preheating zone, moisture is drivenoff from the reducible mixture and the material is heated to atemperature short of substantial fluidization of volatiles in andassociated with the reducible mixture positioned on the hearth cars 86.The design is to reach in the drying/preheating atmosphere a temperaturein the reducible mixture as high as reasonable for removing moisture andheating of the material, but below the temperature of substantialfluidization of the volatiles in the carbonaceous material in andassociated with the reducible mixture. This temperature is generally inthe range of about 200-400° F. (about 95-200° C.), and is selectedusually depending in part on the particular composition of the reduciblemixture and the particular composition of the carbonaceous material. Oneor more preheating burners 92 may be provided in the drying/preheatingzone, for example, in the side walls of the furnace housing 62. Thepreheating burners 92 may be oxy-fuel burners or air/natural gas firedburners as desired, depending on the desired composition of the gas fromthe drying/preheating zone.

The conversion zone 74 is characterized by heating the reducible mixtureto drive off remaining moisture and most of the remaining volatiles inthe reducible mixture, and at least partially reduce the reduciblematerial. The heating in the conversion zone 74 may initiate thereduction reaction in forming the reducible material into metallic ironnodules and slag. The conversion zone 74 is generally characterized byheating the reducible mixture to about 1800 to 2350° F. (about 980° C.to about 1290° C.), or higher, depending on the particular compositionand form of reducible material of the particular embodiment.

The fusion zone 76 involves further heating the reducible mixture, nowabsent most volatile materials, to reduce and melt the iron bearingmaterial, to form metallic iron nodules and slag. The meltingtemperature of the reduced iron is lowered as the amount of carbon inthe iron increases by carburization. The fusion zone generally involvesheating the reducible mixture to about 2400 to 2650° F. (about1310-1450° C.), or higher, so that metallic iron nodules 70 are formedwith a low percentage of iron oxide in the metallic iron. If the methodis carried out efficiently, there will also be a low percentage of ironoxide in the slag, since the method is designed to reduce very highpercentage of the iron oxide in the reducible mixture to metallic ironnodules.

Burners 94 may be provided in the side wall 68 of the furnace housing 62such as shown in FIG. 1 for heating the reducible mixture in theconversion zone 74 and fusion zone 76. Alternatively or in addition, theburners may be positioned in the roof 66 of the furnace housing 62. Theburners 94 are positioned to provide for efficient combustion of thefluidized volatile materials in the conversion zone and to efficientlyreduce the reducible material to NRI in the fusion zone 76. The burners94 should be positioned to provide for efficient heat transfer andefficient reduction of the iron oxide in the reducible mixture with theleast energy consumption. The burners 94 may be positioned on about 10foot centers (about 3 m), staggered along opposite side walls 68, abouta foot down from the roof 66 of the furnace housing 62. Alternatively,or in addition, the burners may be positioned opposite each other in theside walls 68 and/or in the roof 66 of the furnace housing 62. Theburners 94 may be oxy-fuel burners. Alternatively, the burners 94 may beair-fuel burners.

Alternatively, the heating may be carried out in any suitable manner atany suitable temperature. It will be understood that the heating isgenerally carried out in such a manner as to cause fusion or melting ofthe metallic iron produced by the process in the fusion zone. Forexample, the heating may be carried out in an atmosphere using a linearor rotary furnace wherein the conversion zone comprises more than onezone. In the experimental data presented in FIG. 13 described below, thelinear hearth furnace included a first heating zone, or zone 1, whereinthe temperature of the reducible mixture is raised and some reduction ofthe reducible material occurs, and a second heating zone, or zone 2,where further reduction occurs but where the temperature does not exceedthe melting point of iron. The fusion zone, or zone 3 in the data ofFIG. 13, immediately follows the conversion zone and includestemperatures where fusion of the reducible material of the heatedreducible mixture may occur. Alternatively, the fusion zone 76 maycomprise more than one zone. It will be understood however that theheating may occur in any suitable heating atmosphere at any suitabletemperature. In the above example, the first heating zone may have atemperature of up to about 2200° F. (about 1200° C.), the second heatingzone may have a temperature up to about 2400° F. (about 1315° C.), andthe fusion zone may have a temperature up to about 2650° F. (about 1450°C.).

A first baffle 100 may be provided between the drying/preheating zone 78and the conversion zone 74. The first baffle 100 is capable ofinhibiting direct fluid communication between the atmosphere of theconversion zone 74 and the atmosphere of the drying/preheating zone 78.The first baffle 100 may be made of a suitable refractory material, suchas silicon carbide, and may extend downwardly to within a few inches ofthe reducible mixture on the hearth 64. The design is to provide forefficient inhibiting of the direct fluids communication between theconversion zone 74 and the drying/preheating zone 78 in the furnace 60,without interfering with movement of reducible mixture on hearth 64through furnace housing 62.

Optionally, a second baffle 102, such as shown in FIG. 1, may beprovided either between the conversion zone 74 and the fusion zone 76 orpart way into the fusion zone 76. The second baffle 102 is capable ofinhibiting direct fluid communication between the atmosphere of thefusion zone 76 and the atmosphere of the conversion zone 74 wheredesired. The second baffle 102 may be a refractory material, such assilicon carbide, and extend to within a few inches of the heatedreducible mixture positioned on the hearth 64 as it moves through thefurnace housing 62, to effectively inhibit the direct fluidcommunication across the second baffle 102.

The cooling zone 80 provides cooling to reduce the temperature of themetallic iron material 70 from its formation temperature in theconversion zone 74 and fusion zone 76 to a temperature at which themetallic iron material can be reasonably handled and further processed.This temperature after cooling is generally below 800° F. (about 425°C.) and may be below about 500° F. (about 260° C.) or below. The coolingcan be achieved by injection of nitrogen or carbon dioxide throughnozzles 104 in the roofs and/or side walls of the furnace housing 62 orexternal the furnace housing 62. As to the latter, water spray 106 maybe used external the furnace housing 62 for the cooling in the coolingzone 80, if desired and provision made for water handling within thesystem. Alternatively or additionally, a system of coolant tubes 108 maybe positioned over the moving hearth 64 as shown in FIG. 1. A vent hood110 may be positioned above the moving hearth 64 to remove evaporatedwater and other fluidized materials that come off of the hearth duringthe spray cooling.

The cooling zone 80 is optionally in the furnace housing 62. However, itis more desirable in certain embodiments to perform the cooling of themetallic iron material outside the furnace housing 62, such as shown inFIG. 1, to reduce furnace costs, provide for more efficient cooling, andmaintenance and handling considerations.

The exit end of the hearth furnace 60, at the cooling zone 80, may be atleast partially closed by a restricting baffle 112 that inhibits fluidflow between the atmosphere of the fusion zone 76 and the atmosphere ofthe cooling zone 80, yet provides clearance so as not to inhibit themovement of the heated reducible mixture out the furnace housing 62. Thebaffle 112 may be made of a suitable refractory material, such assilicon carbide, and may extend to within a few inches of the heatedreducible mixture positioned on the hearth 64 as the heated reduciblemixture moves through the furnace housing 62.

An exhaust gas system may include an exhaust stack 114 having an inlet116 provided in the conversion zone 74 and/or fusion zone 76. FIG. 1shows the exhaust stack 114, for example, in the fusion zone.Alternatively, the exhaust stack 114 may be positioned in or adjacentthe conversion zone 74 to enable combustion of volatile matter fluidizedin the conversion zone prior to exiting the furnace. The exhaust gassystem may have a variable flue damper, not shown. An in-line damper orpressure control may be provided to control the flue gas stream andimprove zone pressure control. The exhaust gas system may include athermal oxidizer to process the flue gas. Optionally, the flue gas maybe directed to a heat recovery system or other downstream processing.The drying/preheating zone may include a drying zone exhaust stack 118provided to remove moisture and other fluids from the drying/preheatingzone 78. The drying zone exhaust stack 118 may direct the flow from thedrying/preheating atmosphere to combine with the stack gas throughexhaust stack 114 into an exhaust gas system. Alternately, the flow fromthe drying/preheating zone 78 may be directed to a scrubber, baghousefilter, or other exhaust processing separate from the exhaust gassystem.

With reference to FIG. 3, the preparation of the reducible mixture ofiron bearing material and carbonaceous material for processing by thehearth furnace is illustrated. A hearth material layer 120 may beprovided on hearth 64 that includes at least one carbonaceous material.The carbonaceous material may be any carbon-containing material suitablefor use as a reductant with the iron-bearing material. The hearthmaterial layer 120 includes coke, char, or other carbonaceous material,or mixtures thereof. For example, anthracite coal, bituminous coal,sub-bituminous coal, coke, coke breeze or char materials may be used forthe hearth material layer 120. We have found that certain bituminous(e.g. high and medium-volatile bituminous) and sub-bituminous coals maybe used in mixtures with anthracite coal, coke, coke breeze, graphite,or char materials.

The hearth material layer 120 may comprise a mixture of finely dividedcoal and a material selected from the group of coke, char, and othercarbonaceous material found to be beneficial to increase the efficiencyof iron reduction. The coal particles may be a mixture of differentcoals such as non-coking coal, or non-caking coal, sub-bituminous coal,or lignite. The hearth material layer 120 may, for example, includesub-bituminous coal and/or char. Additionally, although up to onehundred percent coal is contemplated for use as a hearth material layer,in some embodiments the finely divided coal may comprise up totwenty-five percent (25%) and mixed with coke, char, anthracite coal, orother low-volatile carbonaceous material, or mixtures thereof. In otherembodiments, up to fifty percent (50%) of the hearth material layer maycomprise coal, or up to seventy-five percent (75%) of the hearthmaterial layer may comprise coal, with the remaining portion coke, char,other low-volatile carbonaceous material, or mixtures thereof. Thebalance will usually be determined by the amount of volatiles desired inthe reduction process and the furnace.

The hearth material layer 120 may comprise two or more layers ofcarbonaceous materials as desired. The hearth material layer 120 mayinclude a first layer of undevolutized coal and a second layer of cokeor char above the first layer of coal. For example, the hearth materiallayer 120 may include a first layer of sub-bituminous coal, and a secondlayer of char material over the coal layer. The char material may bedevolatilized carbonaceous material removed from the hearth at the exitend of the furnace and recycled in the hearth material layer 120 or usedas recycled char for the reducing material in the briquettes asdiscussed below. The layer of char or coke over the layer ofdevolatilized coal slows and extends the fluidization of volatiles fromthe coal as the hearth cars 86 move through the conversion zone 74 tolater stages in the reduction reaction.

The hearth material layer 120 may be of a thickness sufficient toprevent slag from penetrating the hearth material layer 120 andcontacting refractory material of hearth 64. For example, thecarbonaceous material may be ground or pulverized to an extent such thatit is fine enough to prevent the slag from such penetration, buttypically not so fine as to create excess ash. As recognized by oneskilled in the art, contact of slag with the hearth 64 during themetallic iron nodule process may produce undesirable damage to therefractory material of hearth 64. A suitable particle size for thecarbonaceous material of the hearth layer is less than 4 mesh anddesirably between 4 and 100 mesh, with a reasonable hearth layerthickness of about ½ inch or more, is effective protection for thehearth 64 from penetration of the slag and metallic iron duringprocessing. Carbonaceous material less than 100 mesh may be avoidedbecause generally high in ash and resulting in entrained dust that isdifficult to handle in commercial operations. The mesh size of discreteparticles is measured by Tyler Mesh Size for the measurements givenherein.

In some applications, the hearth material layer 120 may be of sufficientthickness to reduce contact adhesion of the iron and slag with therefractory, such as thickness less than ½ inch. In one example, thehearth material layer 120 thickness may be less than 1/16 inch.

The reducible mixture 122 is positioned over the hearth cars 86 above atleast a portion of the hearth material layer, typically prior toentering the furnace. The reducible mixture 122 is generally in the formof a mixture of finely divided iron ore or other iron oxide bearingreducible material, and a carbonaceous reducing material, such as coke,char, anthracite coal or non-caking bituminous and sub-bituminous coal.

The method of producing metallic iron nodules may include providing thelayer of reducible mixture 122 on the underlying hearth material layer120 as further shown in FIG. 3. The layer of reducible mixture includesat least a reducible iron-bearing material and reducing material for theproduction of iron metal nodules and slag. As used herein, iron-bearingmaterial and reducible material includes any material capable of beingformed into metallic iron nodules and slag by the described metalliciron nodule process. The iron-bearing material may include iron oxidematerial, iron ore concentrate, taconite pellets, recyclableiron-bearing material, pellet plant wastes and pellet screened fines.Further, such pellet plant wastes and pellet screened fines may includea substantial quantity of hematite. In addition, such iron-bearingmaterial may include magnetite concentrates, oxidized iron ores, steelplant wastes (e.g., blast furnace dust, basic oxygen furnace (BOF) dustand mill scale), red mud from bauxite processing, titanium-bearing ironsands and ilmenites, manganiferous iron ores, alumina plant wastes, ornickel-bearing oxidic iron ores. Also, less expensive iron ores high insilica may be used. Other reducible iron bearing materials may also beused for making the reducible mixture for producing metallic ironnodules used in the processes described herein to produce metallic ironnodules. For example, nickel-bearing laterites and garnierite ores forferronickel nodules, or titanium bearing iron oxides such as ilmenitethat can be made into metallic titanium iron nodules (while producing atitania rich slag).

The iron-bearing material may include recycled micro metallic ironnodules formed in the process of producing metallic iron nodules. Micrometallic iron nodules (called micro-nodules or micro NRI) include smallparticles of agglomerated iron having a size between about 20 mesh andabout 3 mesh as discussed above. Metallic iron nodules less than 20 meshcan also be used depending on the availability of separation andhandling systems to recycle micro nodules.

In one alternative, the reducible mixture may contain mill scalecontaining more than 55% by weight FeO and FeO equivalent, such asdisclosed in International Patent Application PCT/US2010/021790, filedJan. 22, 2010, and incorporated herein by reference.

The iron-bearing reducible material may be finely-ground or otherwisephysically reduced in particle size. As an example, the sizedistribution of mill scale before reducing particle size is shown inFIG. 7. The particle size of the mill scale or mixture of mill scale andsimilar metallurgical waste may be at least 80% less than 10 mesh.Alternatively, the iron-bearing metallurgical waste may be of a particlesize of at least 80% less than 76 mesh. In one alternative, theiron-bearing material may be ground to less than 65 mesh (i.e., −65mesh) or less than 100 mesh (i.e., −100 mesh) in size for processingaccording to the disclosed method of making metallic iron nodules.Larger size particles, however, of iron-bearing material may also beused. For example, pellet screened fines and pellet plant wastes aregenerally approximately 3 mesh (about 0.25 inches) in average size. Suchmaterial may be used directly, or may be reduced in particle size toincrease surface contact of carbonaceous reductant with the iron bearingmaterial during processing. A smaller particle size tends to reducefusion time in the present method as further discussed below.

Various carbonaceous materials may be used in providing the reduciblemixture 122 of reducing material and iron-bearing reducible material.The reducing material may contain at least a material selected from thegroup consisting of anthracite coal, coke, char, bituminous coal andsub-bituminous coal (including various grades of medium-volatile andhigh-volatile bituminous coals), or combinations thereof. For example,eastern anthracite coal and bituminous non-caking coals may be used asthe carbonaceous reductant in at least one embodiment. However, in somegeographical regions, such as on the Iron Range in Northern Minnesota,the use of western sub-bituminous non-caking coal offers an attractivealternative, as such coals are more readily accessible with the railtransportation systems already in place, plus they are generally lowerin cost and lower in sulfur levels. As such, western sub-bituminouscoals may be used in one or more embodiments of the present method asdescribed herein. Alternatively, or in addition, the sub-bituminouscoals may be carbonized, such as up to about 1650° F. (about 900° C.),prior to its use. The high-volatile bituminous coal used for reducingmaterial as described below has a proximate analysis shown in FIG. 6. Inany case, the carbonaceous material in the reducible mixture may containan amount of sulfur in a range from about 0.2% to about 1.5%, and moretypically, in the range of 0.5% to 0.9%.

Optionally, the present method for producing metallic iron nodules mayinclude delivering a coarse carbonaceous material, shown in FIG. 3 ascarbonaceous material 124, having particle greater than 6 or 4 mesh,such as between 6 or 4 mesh and ½ inch, over the reducible mixture. Incertain applications, the optional coarse carbonaceous material 124 mayassist in fusion, reduce sulfur in the NRI, and inhibit reoxidation ofthe reduced material in forming metallic iron nodules, such as disclosedin U.S. patent application Ser. No. 12/359,729, filed Jan. 26, 2009, andU.S. patent application Ser. No. 12/569,176, filed Sep. 29, 2009. Wehave found, however, under certain conditions a carbonaceous overlayermay increase fusion time. The present method provides production of themetallic iron nodules 70 with reduced residence time in the furnacewithout using a cover layer.

If desired, the optional coarse carbonaceous material 124 may bedelivered in a layer over at least some of the reducible mixture 122 asshown in FIG. 3. The coarse carbonaceous material of the overlayer mayhave an average particle size greater than an average particle size ofthe hearth layer and greater than 6 mesh in particle size. In additionor alternatively, the overlayer of coarse carbonaceous material mayinclude discrete particles having a size greater than 4 mesh and in someembodiments, the overlayer of coarse carbonaceous material may havediscrete particles with a size between 4 mesh or 6 mesh and about ½ inch(about 12.7 mm). There may be of course some particles in the coarsecarbonaceous material less than 4 mesh or 6 mesh in size in commerciallymade products, but in this application the substantial majority of thediscrete particles will be greater than 4 mesh or 6 mesh when a coarsecarbonaceous material of particle size greater than 4 mesh or 6 mesh isdesired. Finer particles of carbonaceous material that may be present insome commercially available compositions are tolerated but not desired.The optional coarse carbonaceous material 124 may be selected from thegroup consisting of anthracite coal, bituminous coal, sub-bituminouscoal, coke, char, and mixtures of two or more thereof.

We have found that the amount and size of reducing material in thereducible mixture 122, the internal carbon, may be varied to effect thefusion time of the reducible material, the amount of micro-nodules(micro NRI) and amount of sulfur in the metallic iron nodules. To reducethe generation of micro-nodules, and to accelerate carburization of thereduced iron before fusion, the reducing material in the reduciblemixture may have a bimodal size distribution including an amount of finecarbonaceous material useful in the reduction of the reducible materialand an amount of coarse carbonaceous material useful in carburizing thereduced iron.

The portion of fine carbonaceous material may be a quantity of firstcarbonaceous reducing material of a size less than about 48 mesh of anamount between about 60% and about 95% of a stoichiometric amountnecessary for complete iron reduction of the reducible iron bearingmaterial (i.e. between about 60% and about 95% stoichiometric). Theportion of coarse carbonaceous material may be a quantity of secondcarbonaceous reducing material with an average particle size greaterthan average particle size of the first carbonaceous reducing materialand a size between about 3 mesh and about 48 mesh of an amount betweenabout 20 percent and about 60 percent of a stoichiometric amount ofnecessary for complete iron reduction of the reducible iron bearingmaterial (i.e. between about 20% and about 60% stoichiometric).

When the reducing material in the reducible mixture includes only finecarbonaceous material, such about −65 mesh, −100 mesh, −200 mesh orcombinations thereof, increasing the amount of carbonaceous materialbeyond about 95% of the stoichiometric amount increased the generationof micro-nodules as shown by the experimental test data in TABLE 1. Wehave also found that providing only a coarser carbonaceous material withnormal size distribution slowed the formation of NRI.

TABLE 1 −200 mesh Fusion coal⁽¹⁾ Mix time, Micro NRI NRI % stoich. No.(min) NRI (%) % C % S 115 P-758 4 9.9 2.30 0.031 105 P-757 4 6.5 2.320.032 100 P-790 5 1.2 2.27 0.042 95 P-756 7 0.9 2.23 0.063 90 P-789 70.1 2.80 0.053 85 P-752 8 0.9 2.80 0.054 80 P-788 14 0.2 2.83 0.094 75P-752 14 0.9 2.59 0.103 80 P-788 13 0.5 2.86 0.067 75 P-787 15 0.6 2.520.096 70 P-857 >20  —⁽²⁾ — — ⁽¹⁾High-volatile bituminous coal. ⁽²⁾Notfused.

Table 1 is a summary of the effects of the reducible mixture havingdifferent amounts of high-volatile bituminous coal at −200 mesh, with 2%fluorspar and a slag composition for basicity B₂ of 1.5, briquetted with4% molasses as a binder, placed on 6/100 mesh anthracite char hearthlayer, and heated at 1400° C. (2552° F.) for different periods of timein a box furnace N₂—CO atmosphere. The compositions of the reduciblemixtures used in Table 1 are shown in FIG. 8. The composition of themill scale is shown in FIG. 5, and size distribution as received shownin FIG. 7. As used herein, basicity B₂ is the ratio of CaO/SiO₂, andbasicity B₄ is the ratio of CaO+MgO/SiO₂+Al₂O₃.

When the reducing material in the reducible mixture includes a bimodalsize distribution, the fusion time may be reduced. Table 2 is a summaryof preliminary tests in the box furnace using bimodal size distributionof carbonaceous material in the reducible mixture. The reduciblematerial in the mixture was mill scale. The test of Table 2 includes avaried amount of anthracite char recycled from a prior heating in makingNRI or DRI (recycled anthracite) having a size of 6/28 mesh with 85% ofthe stoichiometric amount of high-volatile bituminous coal at −200 mesh.The reducible mixture included 2% fluorspar and a slag composition forbasicity B₂ of 1.5. As shown by Table 2, the bimodal distribution ofcarbonaceous material in the reducible mixture reduced the fusion timeby about 50% without significantly increasing the generation ofmicro-nodules. The amount of sulfur in the NRI produced was alsolowered.

TABLE 2 Recycled Fusion anthracite time at Micro 6/28 mesh 1400° C. NRINRI (% stoichiometric) (min) (%) % S 0 8 1.8 0.093 40 4 1.4 0.053 50 43.4 0.056 60 4 1.4 0.060

The effect of using bimodal size distribution was tested in a linearhearth furnace providing results summarized in Table 3 with mill scaleas the reducible material. The test of Table 3 includes a varied amountof anthracite char recycled from a prior heating in an NRI process(recycled anthracite) having a size of 6/28 mesh with 85% of thestoichiometric amount of high-volatile bituminous coal at −200 mesh. Thereducible mixture included 2% fluorspar and a slag composition forbasicity B₂ of 1.5. Additionally, in the test of Table 3 a cover layerwas provided over the reducible mixture using 1.5 lb/ft² of anthracite.

TABLE 3 Recycled Fusion anthracite time at Micro 6/28 mesh 1400° C. %NRI NRI (% stoichiometric) (min) Fused (%) % S 0 70 100 3.3 0.107 40 51100 — — 56 100 7.6 0.082 50 56 100 8.5 0.068 51 95 — — 60 51 100 — — 51100 14.6 0.065

The first carbonaceous reducing material and second carbonaceousreducing material provide total amount of carbonaceous reducing materialin the reducible mixture, or total reducing material carbon that may bebetween about 100% and 150% of the stoichiometric amount of necessaryfor complete iron reduction of the reducible iron bearing material.Alternatively, the first carbonaceous reducing material and secondcarbonaceous reducing material provide total reducing material carbonbetween about 110 and 140 percent of a stoichiometric amount. As shownby the experimental data in Table 4 below, fusion time in a box furnacefor different relative amounts of high-volatile bituminous coal andrecycled anthracite remained nearly the same within the total carbon inthe reducible mixture of 115%, as well as within 125% of thestoichiometric amount. In the experiment for Table 4, the fusion time at125% of the stoichiometric amount was typically 4 minutes, which wassomewhat shorter than at 115% of the stoichiometric amount at 5 minutes.As compared to 85% stoichiometric high-volatile bituminous coal byitself from Table 1, fusion time decreased by as much as 50% when the6/28 mesh recycled anthracite was added without significantly increasingthe amount of micro-nodules produced. Additionally, 115% stoichiometriccoal by itself from Table 1 also provided a fusion time of 4 minutes,but generated about 10% micro-nodules. By providing a bimodal sizedistribution, the amount of micro-nodules may be reduced.

TABLE 4 Coal⁽¹⁾ + Recyc. Fusion Micro Anthracite Mix Time NRI NRI NRI (%stoich.) No. (min) (%) % C % S Total Carbon 115% Stoich: 70% + 45% P-7865 0.3 2.56 0.051 75% + 40% P-775 4 0.3 2.50 0.046 80% + 35% P-777 5 0.02.18 0.052 85% + 30% P-763 5 1.3 2.49 0.051 90% + 25% P-779 5 0.2 2.330.043 95% + 20% P-766 5 3.9 2.42 0.048 Total Carbon 125% Stoich: 75% +50% P-783 4 0.8 2.65 0.047 80% + 45% P-784 5 0.2 2.46 0.049 85% + 40%P-781 4 0.7 2.47 0.035 90% + 35% P-785 4 0.2 2.37 0.042 ⁽¹⁾High-volatilebituminous coal.

The compositions of the reducible mixtures used in Table 4 are shown inFIG. 9. In the test for Table 4, the reducible material was mill scale.The high-volatile bituminous coal was sized to −200 mesh, and therecycled anthracite was 6/28 mesh. The reducible mixture included 2%fluorspar and a slag composition for basicity B₂ of 1.5, briquetted with4% molasses as a binder, placed on 6/100 mesh anthracite char hearthlayer, and heated at 1400° C. (2552° F.) for different periods of timein a box furnace N₂—CO atmosphere.

Referring now to Tables 5 and 6, experimental data from initial testscollected in a test linear hearth furnace are provided. The compositionsof the reducible mixtures used in Tables 5 and 6 are shown in FIG. 10.The test of Tables 5 and 6 was performed without a carbonaceous coverlayer, providing residence times in the furnace as low as 18 minutes. Ina prior test in the linear hearth furnace with high-volatile bituminouscoal alone and a carbonaceous overlayer, the residence time in thefurnace was as high as 70 seconds. By using a bimodal size distribution,the rate of production may be increased without significantly increasingthe generation of micro-nodules.

TABLE 5 Recy. Slag Speed Mix Coal⁽¹⁾ Anth. Basicity setting ResidenceMicro NRI No. % stoich % stoich B₂ B₄ (“/min) (min) NRI % C % S P-831 8530 1.50 1.29 7  (40)⁽²⁾ little 2.03 0.186 P-832 85 40 1.50 1.28 7 (40)little 2.29 0.158 P-833 85 50 1.50 1.27 7 (40) little 2.22 0.136 P-83485 60 1.50 1.25 7 (40) little 2.17 0.136 P-903 95 40 1.50 1.27 8 (35)little 2.37 0.158 P-904 105 40 1.50 1.26 9 (31) some 2.43 0.118 P-905115 40 1.50 1.26 9 (31) much 2.36 0.098 P-906 125 40 1.50 1.25 9 (31)much 2.20 0.123 P-900 100 40 1.50 1.27 7 (40) some 2.27 0.147 P-902 10060 1.50 1.24 8 (35) some 2.70 0.115 P-919 100 80 1.50 1.24 9 (31) some2.51 0.100 P-920 100 100 1.50 1.22 9 (31) some 2.28 0.118 P-935 100 401.70 1.42 12  27⁽³⁾ some 2.60 0.110 P-936 100 40 1.90 1.57 12 27 some2.88 0.079 P-937 100 60 1.70 1.39 12 27 some 2.67 0.090 P-938 100 601.90 1.54 12 27 some 2.99 0.074 P-929 90 40 1.50 1.27 12 30 little 2.540.133 P-930 90 50 1.50 1.26 12 30 little 2.40 0.140 P-931 90 60 1.501.25 12 30 little 2.52 0.146 P-932 90 70 1.50 1.24 12 30 little 2.540.146 P-939 90 40 1.70 1.43 10 32 little 2.24 0.130 P-940 90 40 1.901.58 10 32 little 2.67 0.099 P-941 90 60 1.70 1.40 10 32 little 2.370.132 P-942 90 60 1.90 1.55 10 32 little 2.72 0.090 ⁽¹⁾High-volatilebituminous coal. ⁽²⁾Fusion time was not reached at these settings.⁽³⁾Actual timer reading at fusion time

TABLE 6 Recy. Slag Speed Mix Coal⁽¹⁾ Anth. Basicity setting ResidenceMicro NRI No. % stoich % stoich B₂ B₄ (“/min) (min) NRI % C % S P-951100 40 2.10 1.72 12  22.5⁽³⁾ some 2.70 0.070 P-952 100 40 2.30 1.88 1619 some 3.11 0.045 P-953 100 60 2.10 1.69 12 22.5 some 3.06 0.050 P-954100 60 2.30 1.84 18 18.5 some 3.25 0.039 P-947 110 40 1.70 1.41 12 24.5more 3.52 0.090 P-948 110 40 1.90 1.57 14 22.5 more 2.43 0.080 P-949 11060 1.70 1.38 12 24.5 more 2.74 0.088 P-950 110 60 1.90 1.53 14 22.5 more2.69 0.067 P-955 100  40⁽²⁾ 1.90 1.57 14 21 little 2.89 0.066 P-956 100 40⁽²⁾ 2.10 1.72 16 19 little 3.22 0.044 P-957 100  60⁽²⁾ 1.90 1.54 1818 little 2.90 0.056 P-958 100  60⁽²⁾ 2.10 1.69 18 18 little 3.08 0.039P-959 100 40 2.50 2.03 16 19.5 little 3.23 0.035 P-960 100 40 2.70 2.1916 19.5 little 3.55 0.030 P-961 100 60 2.50 1.99 14 20 little 3.78 0.029P-962 100 60 2.70 2.15 14 20 little 3.69 0.027 ⁽¹⁾High-volatilebituminous coal. ⁽²⁾Fluorspar increased to 4%. ⁽³⁾Actual timer readingat fusion time

As an example, FIG. 11 shows test samples from Table 5 prior to heating.In the upper left quadrant are samples of P-831 having 30% recycledanthracite. The upper right has samples of P-832 having 40% recycledanthracite. In the lower left are samples of P-833 having 50% recycledanthracite, and the lower right has samples of P-834 having 60% recycledanthracite. FIG. 12 shows the test sample of FIG. 11 after heating withfew micro-nodules present.

As shown in Tables 5 and 6, increasing the slag basicity B₂ to greaterthan about 2.1 may increase the rate of fusion reaction and decrease NRIsulfur. In the present method, the basicity B₂ may be between about 1.5and 2.7, and basicity B₄ may be between about 1.2 and 2.2. In oneapplication, the reducible mixture may include 100% stoichiometrichigh-volatile bituminous coal at −100 mesh and 60% stoichiometricrecycled anthracite at 6/28 mesh, with a slag composition for basicityB₂ of higher than about 2.3 and B₄ higher than about 1.8. A partialsample of experimental test parameters used in the test for Tables 5 and6 is shown in FIG. 13.

As shown in Tables 5 and 6, fluorspar may be provided in the reduciblemixture to decrease the residence time. In the present experiments, theresidence time is the time from the entry of the furnace to the exit.The fluorspar addition may be between about 1 and 4%.

The bimodal size distribution of carbonaceous material in the reduciblemixture includes an amount of first reducing material useful in thereduction of the reducible material and an amount of second reducingmaterial useful in the reduction of the reducible material, where theaverage particle size of the second reducing material is greater thanaverage particle size of the first reducing material. The firstcarbonaceous reducing material has an average particle size that issmaller than the average particle size of the second carbonaceousreducing material. In one application, the first carbonaceous reducingmaterial is about −28 mesh. Alternatively, the first carbonaceousreducing material may be about −35 mesh. Alternatively, the firstcarbonaceous reducing material may be about −48 mesh. Alternatively, thefirst carbonaceous reducing material may be about −65 mesh.Alternatively, the first carbonaceous reducing material may be about−100 mesh.

In one application, the second carbonaceous reducing material is betweenabout 48 mesh and 3 mesh (i.e. 3/48 mesh). Alternatively, the secondcarbonaceous reducing material may be between about 48 mesh and 6 mesh(i.e. 6/48 mesh). Alternatively, the second carbonaceous reducingmaterial may be between about 48 mesh and 8 mesh (i.e. 8/48 mesh).Alternatively, the second carbonaceous reducing material may be betweenabout 48 mesh and 10 mesh (i.e. 10/48 mesh). Alternatively, the secondcarbonaceous reducing material may be between about 48 mesh and 14 mesh(i.e. 14/48 mesh). Alternatively, the second carbonaceous reducingmaterial may be between about 48 mesh and 28 mesh (i.e. 28/48 mesh).

In other applications, the smaller size screen for the alternatives ofthe preceding paragraph for the second carbonaceous reducing materialmay be 28 mesh instead of 48 mesh, or may be 20 mesh instead of 48 mesh.In one example, the reducible material is finely ground taconite and thesmaller screen size is between about 28 and 20 mesh, and the largerscreen size is between about 14 and 6 mesh.

The first carbonaceous reducing material may include a plurality ofcarbonaceous materials, such as a combination of coals and/or char, orother combinations as desired. As shown in Tables 2 through 6, thesecond carbonaceous reducing material may be recycled anthracite. In oneapplication, fines of recycled anthracite, such as −48 mesh, or −65mesh, may be used in combination with coal in the first carbonaceousreducing material.

The data from an initial test including mill scale as the reduciblematerial, different amounts of high-volatile bituminous coal, 60%stoichiometric recycled anthracite of 6/28 mesh, 4% fluorspar, and acomposition for slag basicity B₂ of 2.3 is plotted in FIG. 14 showingthe effect of increasing first carbonaceous reducing material on fusiontime, sulfur and micro-nodule generation. By reducing the amount offirst carbonaceous material, the amount of micro-nodules can be reducedwhile maintaining a low sulfur level in the NRI product.

Table 7 below shows one experiment of the effect of fluorspar additionand basicity B₂ on the fusion time. In this application, the secondcarbonaceous reducing material was anthracite char of 6/28 mesh recycledanthracite and the basicity B₂ between 2.1 and 2.7. The composition ofthe reducible mixture used in a representative portion of the test ofTable 7 is shown in FIG. 15.

TABLE 7 Slag Time in Micro Fluorspar, basicity Mix LHF LHF, NRI, NRI NRI% B₂ No. No. (min) (%) % C % S 85% Stoich. coal⁽¹⁾ and 60% stoich.recycled anthracite 4 2.7 P-984  983 19.6 4.1 3.40 0.024 3 2.5 P-10011018 23.5 5.6 3.15 0.032 3 2.3 P-991 1003 19.6 4.5 2.96 0.037 2 2.7P-985  991 >21.5   — — — 2 2.5 P-1000 1026  (23.5)⁽⁴⁾ 4.1 3.27 0.035 22.3 P-990  995 21.5 3.1 3.46 0.034 85% Stoich. coal⁽¹⁾ and 40% stoich.recycled anthracite 4 2.7 P-988  987 19.6 — — — 3 2.5 P-1003 1020 23.05.5 3.18 0.036 3 2.3 P-997 3 2.1 P-993 1009  >23.5⁽⁵⁾ — — — 2 2.7 P-989 989 >29⁽³⁾  — — — 2 2.5 P-1002 1028  (27.5)⁽⁴⁾ 6.7 3.37 0.044 2 2.3P-996 1016 24.5 5.4 3.21 0.060 2 2.1 P-992 1006  (23.5)⁽⁴⁾ 2.0 2.980.062 2 2.1 P-1033 1069 33.5 7.0 0.062 2 1.9 P-1034 1070 38   4.9 0.0972 1.7 P-1035 1071 50.5 — 0.139 2 1.5 P-1036 1072 50.5 — 0.106 75%Stoich. coal⁽¹⁾ and 60% stoich. recycled anthracite 3 2.5 P-1005 3 2.3P-999 1034 25.5 3.7 3.02 0.049 3 2.1 P-995 1011  (26.5)⁽⁴⁾ 5.8 3.230.053 2 2.5 P-1004 1027 26.5 3.7 3.52 0.032 2 2.3 P-998 2 2.1 P-994 1010 (26.5)⁽⁴⁾ 6.3 3.29 0.051 ⁽¹⁾High-volatile bituminous coal. ⁽²⁾About 50%fused. No further tests. ⁽³⁾Longer residence time due to testinterruption. About 50% fused. No further tests. ⁽⁴⁾62% of the normalweight used, as the briquettes exhausted. ⁽⁵⁾75% of the normal weightused, as the briquettes exhausted. About 67% fused. No further tests.

The amount of carbonaceous reducing material in the mixture with ironbearing material to form the reducible mixture 122 may vary dependingupon the percentage of iron in the iron-bearing reducible material, thesources of reducible material and carbonaceous reducing material, thecarbonaceous reducing material, the furnace used, as well as the furnaceatmosphere maintained in which the reducing reaction takes place. Insome embodiments, where the iron bearing material is hematite ormagnetite or mixtures thereof, the amount of first carbonaceous reducingmaterial in the reducible mixture may be less than where the ironbearing material in the reducible mixture is mill scale or the like withhigh levels of FeO.

The amount of first carbonaceous reducing material may be between about65% and 95% stoichiometric. In one example, such as for mill scale asthe reducible material, the amount of first carbonaceous reducingmaterial may be between about 85% and 95% stoichiometric. Alternatively,such as for magnetite as the reducible material, the amount of firstcarbonaceous reducing material may be between about 80% and 90%stoichiometric. In yet another alternative, such as for hematite as thereducible material, the amount of first carbonaceous reducing materialmay be between about 65% and 75% stoichiometric. The amount of secondcarbonaceous reducing material may be between about 20% and 60%stoichiometric. Alternatively, the amount of second carbonaceousreducing material may be between about 30% and 50% stoichiometric.

The effect of the size of the coarse portion, or second carbonaceousmaterial, in the reducible mixture is shown in Table 8. As shown inTable 8, reducing the size of the second carbonaceous material tends toincrease the amount of micro-nodules.

TABLE 8 Recycled Anthracite Time in Size LHF Micro NRI % stoich (mesh)(min) NRI (%) % S 40  6/28 24.5 5.4 0.060 10/20 20/35 22.5 5.3 0.04035/65 (22.5) 7.5 0.038 60  6/28 21.5 6.2 0.034 10/20 21.5 5.0 0.03820/35 20.5 7.1 0.037 35/65 (21.5) 14.0 0.041

A smaller particle size of the reducible material tends to reduce fusiontime in the present method. The effect of various particle sizes of millscale is shown in Tables 9 and 10. In Table 9, mill scale ground to −20,−35, and −100 was compared in a box furnace, and in Table 10 thereducible mixture was processed in a linear hearth furnace. The sizedistribution of mill scale as received is shown in FIG. 6. In both thebox furnace and the linear hearth furnace, the reduction of particlesize decreased the fusion time. Additionally, the sulfur content in theiron nodules also decreased with reduction of particle size. But theamount of micro-nodules increased. A reduction of carbonaceous materialin the reducing mixture may reduce the micro nodules as discussed above.

TABLE 9 Fusion Micro time, NRI, NRI Grind (min) (%) % S As received 40.6 0.038 −20 mesh 3.5 3.5⁽¹⁾ 0.024 4 1.2 0.027 −35 mesh 3.5 3.8 0.023−100 mesh  3 6.1 0.018 ⁽¹⁾Nearly fully fused.

TABLE 10 Time in Micro LHF, NRI, NRI Grind (min) (%) % S As received24.5 5.4 0.060 −20 mesh 21 8.2 0.044 −35 mesh 19.5 9.6 0.033 −100 mesh 18.5⁽¹⁾ 16.3 0.030 ⁽¹⁾Stalled for 1.5 minutes in zone 1.

In one alternative, the reducible material may be taconite. The effectof varying basicity B₂ with the reducible material being taconite fromone series of experiments is summarized in Table 11. The test of Table11 includes taconite concentrate, 85% stoichiometric medium-volatilebituminous coal for first carbonaceous material and 40% stoichiometricrecycled anthracite for second carbonaceous material. As shown in Table11, the amount of micro-nodules increased as basicity B₂ increased. Thecomposition of the reducible mixture of the test of Table 11 is shown inFIGS. 16A and 16B.

TABLE 11 Al₂O₃ Fusion micro Mix in slag time NRI NRI B2 No. (%) (min)(%) % S 2.3 P-1083 11 3.5 17.8 0.015 P-1081 15 3.5 12.9 0.020 2.1 P-108811 3 11.8 0.016 P-1087 15 3 5.9 0.017 1.9 P-1084 11 3 8.9 0.017 P-108215 3 3.0 0.024 1.7 P-1090 11 2.5 1.9 0.024 P-1089 15 3 2.7 0.022 1.5P-1093 11 3 2.4 0.031 P-1094 15 3 2.3 0.029

TABLE 12 Fusion time (min) NRI % S Taconite Mill Taconite Mill B₂concentrate Scale concentrate Scale 2.5 — 6 — 0.026 2.3 >15 4 0.0170.038 2.1 9 4 0.018 0.036 1.9 5 4 0.020 0.050 1.7 4 4 0.028 0.053 1.5 55 0.033 0.058

As shown in Table 12, when the reducible material is taconite, withoutfurther adjustment to the slag composition the basicity B₂ may have agreater effect on fusion time as compared to mill scale. With bothtaconite and mill scale, decreasing basicity B₂ lowered the amount ofsulfur in the NRI. We have found that an increase of Al₂O₃ in the slag,such as to 15% in the slag, will reduce the affect that basicity B₂ hason fusion time using taconite.

Additives may optionally be provided to the reducible mixture 122separately or in combination for one or more purposes, in addition tothe reducing material (e.g., coal or char) and reducible iron-bearingmaterial (e.g., iron oxide material or iron ore). For example, additivesmay be provided for controlling slag basicity B₂, as binders and/or toprovide binder functionality (e.g., lime can act as a weak binder forcertain mixtures when wetted), for controlling the slag fusiontemperature, to reduce the formation of micro-nodules, and/or forfurther controlling the content of sulfur in resultant iron nodulesformed by the metallic iron nodule process. The table of FIG. 4 showsthe chemical compositions of various exemplary additives to thereducible mixture 122. These additives include, for example, chemicalcompositions such as Al(OH)₃, bauxite, bentonite, Ca(OH)₂, lime hydrate,limestone, and Portland cement. Other additives may also be used such asCaF₂, Na₂CO₃, fluorspar, soda ash, aluminum smelter slag, cryolite, andSiO₂. Some of the exemplary additives contain trace amounts of Mg, asshown, and in some examples Mg should not be used in quantities thatwill produce 5% mass or more MgO in the resulting slag.

The reducible mixture 122 may be formed into compacts either in situ onthe hearth or preformed as briquettes, balls or extrudates (with orwithout binder) suitable for use in forming metallic iron nodules by thedisclosed process. Compacts refer to any compacted reducible mixturepreformed or formed in situ as any desired discrete profile forpositioning on the hearth layer. For example, discrete portions,compacts, may also be preformed balls or shaped reducible mixtures suchas briquettes or extrudates, which may be preformed using compaction orpressure.

In the present method of making metallic iron nodules, the preparedreducible mixture 122 is heated in a drying/heating atmosphere to driveoff moisture and heat the mixture, and then heated in a reducingatmosphere to drive off remaining moisture, fluidize volatiles in thecarbonaceous materials and at least partially reduce the reduciblemixture. Next, the at least partially reduced reducible mixture isheated in a fusion atmosphere above the melting point of iron to form,one or more metallic iron nodules and slag. As further shown in FIG. 2,resultant slag 126 on hearth material layer 120 is shown with the one ormore metallic iron nodules 70. That is, slag beads on hearth materiallayer 120 are separated from the iron nodules 70 or attached thereto.The metallic iron nodules 70 and slag 126 (e.g., attached slag beads)are discharged from hearth 64, and the discharged metallic nodules arethen separated from the slag 126.

This invention has been described with reference to illustrativeembodiments and is not meant to be construed in a limiting sense. Itwill be apparent to one skilled in the art that elements or processsteps from one or more embodiments described herein may be used incombination with elements or process steps from one or more otherembodiments described herein, and that the present invention is notlimited to the specific embodiments provided herein but only as setforth in the accompanying claims. Various modifications of theillustrative embodiments, as well as additional embodiments to theinvention will be apparent to persons skilled in the art upon referenceto this description.

What is claimed is:
 1. A method for producing metallic iron nodulescomprising the steps of: providing a hearth comprising refractorymaterial; providing reducible mixture above at least a portion of therefractory material, the reducible mixture comprising at least reducingmaterial and reducible iron bearing material; forming a reduciblemixture comprising: a quantity of reducible iron bearing material, aquantity of first carbonaceous reducing material of a size less thanabout 48 mesh of an amount between about 65 percent and about 95 percentof a stoichiometric amount necessary for complete iron reduction of thereducible iron bearing material, and a quantity of second carbonaceousreducing material with an average particle size greater than averageparticle size of the first carbonaceous reducing material and a sizebetween about 3 mesh and about 48 mesh of an amount between about 20percent and about 60 percent of a stoichiometric amount of necessary forcomplete iron reduction of the reducible iron bearing material; wherethe amount of first carbonaceous reducing material and secondcarbonaceous reducing material provide total reducing material carbon atleast 100 percent of a stoichiometric amount necessary for complete ironreduction of the reducible iron bearing material; thermally treating thereducible mixture in the presence of other carbonaceous materialseparate from the reducible mixture to form one or more metallic ironnodules.
 2. The method for producing metallic iron nodules as claimed inclaim 1 where amount of first carbonaceous reducing material and secondcarbonaceous reducing material provide total reducing material carbonbetween about 110 and 140 percent of a stoichiometric amount necessaryfor complete iron reduction of the reducible iron bearing material. 3.The method for producing metallic iron nodules as claimed in claim 1where the quantity of first carbonaceous reducing material being of anamount between about 80 percent and about 90 percent of a stoichiometricamount necessary for complete iron reduction of the reducible ironbearing material.
 4. The method for producing metallic iron nodules asclaimed in claim 1 where the quantity of first carbonaceous reducingmaterial being of an amount between about 85 percent and about 95percent of a stoichiometric amount necessary for complete iron reductionof the reducible iron bearing material.
 5. The method for producingmetallic iron nodules as claimed in claim 1 where the quantity of firstcarbonaceous reducing material being of an amount between about 65percent and about 75 percent of a stoichiometric amount necessary forcomplete iron reduction of the reducible iron bearing material.
 6. Themethod for producing metallic iron nodules as claimed in claim 1 wherethe quantity of second carbonaceous reducing material being of an amountbetween about 20 percent and about 50 percent of a stoichiometric amountnecessary for complete iron reduction of the reducible iron bearingmaterial.
 7. The method for producing metallic iron nodules as claimedin claim 1 where the first carbonaceous reducing material is a size lessthan about 65 mesh.
 8. The method for producing metallic iron nodules asclaimed in claim 1 where the first carbonaceous reducing material is asize between about 65 mesh and about 100 mesh.
 9. The method forproducing metallic iron nodules as claimed in claim 1 where the secondcarbonaceous reducing material is a size between about 48 mesh and about6 mesh.
 10. The method for producing metallic iron nodules as claimed inclaim 1 where the first reducing material is a carbonaceous materialwith between 2 and 40% average volatiles.
 11. The method for producingmetallic iron nodules as claimed in claim 1 where the second reducingmaterial is a non-coking carbonaceous material with less than 10%average volatiles.
 12. The method for producing metallic iron nodules asclaimed in claim 1 where the second reducing material is a non-cokingcarbonaceous material with between 1 and 8% average volatiles.
 13. Themethod for producing metallic iron nodules as claimed in claim 1 furthercomprising the step of: prior to the step of thermally treating thereducible mixture, forming the reducible mixture into agglomerates. 14.The method for producing metallic iron nodules as claimed in claim 1where the second carbonaceous reducing material is a size less than 20mesh and further comprising the step of: prior to the step of thermallytreating the reducible mixture, forming the reducible mixture intoballs.
 15. The method for producing metallic iron nodules as claimed inclaim 1 where the first carbonaceous reducing material includes at leasttwo sources of carbonaceous material, at least one source being finesless than about 48 mesh from a source of carbonaceous material in thesecond carbonaceous reducing material.
 16. The method for producingmetallic iron nodules as claimed in claim 1 where the basicity B₂ of thereducible mixture, defined as the ratio of CaO/SiO₂ in the mixture, isbetween 1.5 and 2.3.
 17. The method for producing metallic iron nodulesas claimed in claim 1 where the basicity B₂ of the reducible mixture,defined as the ratio of CaO/SiO₂ in the mixture is between 1.9 and 2.3.